1. Field of the Invention
The present invention relates to a spectacle lens for correcting eyesight.
2. Description of the Related Art
It is desirable for a spectacle lens used for correcting eyesight (near-sightedness, far-sightedness, astigmatism, etc.) to be formed such that the curvature of the front surface of the lens is gentle (i.e., the curvature of the front surface of the lens is small), and that both the central and peripheral thicknesses of the lens are small, while maintaining satisfactory optical performance. The reason for this is that in recent years great importance has been paid to the appearance of the lens and the comfort in wearing the lens. It is also desirable that the spectacle lens can be easily made so as to reduce the cost of production.
In a conventional spectacle lens having a spherical surface or a toric surface, the curvature of the front surface of the lens becomes somewhat sharp (i.e., the curvature of the front surface of the lens becomes large) when greater importance is placed on the optical performance of the lens, rather than the appearance of the lens or the comfort in wearing the lens. Specifically, in such a case, the central thickness of the lens or the peripheral thickness of the lens becomes large when the conventional spectacle lens is a negative lens or a positive lens, respectively. Therefore, such a conventional spectacle lens is heavier and less comfortable when greater importance is placed on the optical performance of the lens. Conversely, in the case where the front surface of the lens is formed to have a gentle curvature so as to improve the appearance of the lens or the comfort in wearing the lens, optical performance deteriorates.
In order to improve the appearance of the lens and the comfort in wearing the lens while maintaining the optical performance of the lens, a lens having a front aspherical surface to reduce the curvature of the front surface of the lens while maintaining satisfactory optical performance of the lens has been recently proposed. Such an aspheric lens is disclosed in each of U.S. Pat. Nos. 5,050,979, 5,050,980 and 5,270,746.
Each of U.S. Pat. Nos. 5,050,979 and 5,050,980 discloses a spectacle lens provided at its front with an aspherical surface which is rotationally symmetrical (i.e., rotationally-symmetrical aspherical surface). In the case where the front surface of a spectacle lens is formed as a rotationally-symmetrical aspherical surface, the curvature of the front surface becomes more gentle compared with that of the front surface of a spectacle lens which is formed as an optimum spherical surface. The lens appearance thus improves while the thickness and the weight of the lens are reduced.
However, such a conventional spectacle lens provided with the rotationally-symmetrical aspherical surface at its front does not exhibit excellent optical performance over the entire object distance range. Namely, such a conventional spectacle lens can be designed to exhibit an excellent optical performance only over a limited object distance range. FIGS. 9, 10 and 11 show an example of such a conventional spectacle lens (example 1), and FIGS. 12, 13 and 14 show another example of the same (example 2).
In FIG. 9, reference numerals 47, 57, 27 and 37 designate the front surface, the rear surface, the intersection of the rotational axis of the lens (which extends through the geometric center thereof with the front surface) and the center of the aspherical surface, respectively. Likewise, in FIG. 12, reference numerals 48, 58, 28 and 38 designate the front surface, the rear surface, the intersection of the rotational axis of the lens (which extends through the geometric center thereof with the front surface) and the center of the aspherical surface, respectively. In each example, the intersection 27 or 28 is coincident with the center of the aspherical surface 37 or 38, as can be seen from FIG. 9 or 12. Lens data regarding these examples is shown below.